Various plasma jet devices, such as those which employ a direct current and an ionized gas, are useful for generating a plasma jet for spectrometric analysis or for studies of high-temperature chemical and physical phenomena of various materials. In particular, such plasma jet devices are often employed in a spectrometer system, such as in an echelle spectrometer of the type described in U.S. Pat. No. 3,658,423, wherein a prism and echelle grating are employed and so mounted to provide rotation in two directions, thereby providing adjustment of the vertical and horizontal components of the dispersed energy in the exit focal plane of the spectrometer. Of course, plasma jet devices may be employed usefully in other spectrometers and in other apparatus where high-temperature excitation of a sample material is desired.
One plasma jet or excitation source useful, for example, in spectroscopic analysis, is described in U.S. Pat. No. 3,596,128. Such a plasma jet device includes a swirl chamber surrounding an anode electrode, into which swirl chamber a premixed atomized sample to be observed and an ionizing carrier gas are introduced. An anode is disposed in the chamber and located opposite an orifice. A cathode is located externally to and spaced from said chamber and at an angle to the axis of the plasma column, so that the cathode is offset from said plasma column. The plasma flame, after exiting from the swirl chamber through the orifices, is bent at an angle to the axis of the plasma column to contact said cathode electrode. This plasma device, while representing an improvement over the prior art, presents certain difficulties associated with the construction of the device.
An improvement of this plasma jet device is described in U.S. Pat. No. 4,009,413, and is hereby incorporated by reference in its entirety. The improved plasma jet device comprises an anode electrode and a cathode electrode, with each of the electrodes spaced apart and positioned such that their axes, if extended, would intersect at an angle. Each of the electrodes contains a coaxial sleeve element surrounding the electrodes through which flows an ionizable gas. The ionizable gas in operation forms the plasma jet and provides a continuous column of ionized gas between the anode and cathode electrodes, the plasma jet being characterized in such form by an inverted V-form shape. The plasma jet presents a reaction or excitation zone within the plasma jet at the lower region of the intersection of the extended axis of the anode and cathode electrodes. The plasma jet also includes an external means to introduce a sample, typically in aerosol form, and particularly in an ionizable carrier gas, upwardly between the anode and cathode, so that the sample is introduced directly into the reaction or excitation zone of the plasma arc.
In these prior-art plasma arc devices, certain difficulties in operation of the devices have been found in that the position of the plasma arc tends to move about, and, therefore, the excitation or reaction zone moves in position. With such movement of the excitation zone, there is a variation and difference in intensity of the spectrum from the sample being introduced into the zone, and, a difference in the quality and quantity of the spectrometric data. The spectrometric data obtained changes with such destabilization of the plasma arc position.
In addition, the prior-art devices have required employment of tungsten anodes. The use of a tungsten anode, rather than, for example, a graphite anode in combination with a tungsten cathode, creates certain difficulties in that the spectral level of tungsten is very high, and provides for a substantial number of up to 4,000 interfering tungsten lines in the spectrum, which may cause interference sometimes with the analysis of the spectrometric data obtained. In contrast, the employment of the graphite anode provides only a few interfering lines in the spectrum.